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I am replacing the Regulator Rectifier on a John Deer tractor and there are 6 terminals.

Bat + (Battery positive)
IGN (Ignition switch, engages the regulator)
Bat - (Battery Negative)
AC (AC Input from the Stator)
L (Warning Light?)
AC (AC Input from the Stator)

What is the L terminal used for, my last RR burnt to a crisp but only has 5 wires (which I've identified), would it be reasonable to assume that disconnecting one of the AC inputs would trigger the L connection to close and then I can test for Pos/Neg?

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Normally, if the L terminal is for the light, then it will be positive when the engine is running and the alternator charging.

The other side of the charge warning light is also connected to battery positive so if the voltages are the same then the light will be out. If the alternator is not charging at all, or not enough, then the voltage from the battery exceeds the alternator voltage and the light will glow or shine brightly depending what is happening.

This is usually termed a voltage balance. So this means the L terminal can be positive or negative, well battery negative, depending what is happening.

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  • Thanks, I'll use my multimeter to confirm and wire up a 12v LED to my dash. Jul 9, 2020 at 17:02
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    You cannot use a 12v LED as they are polarity sensitive ie only work one way, you will need an "ordinary" small bulb some are 5W some smaller. Well, you could use an LED with a dedicated circuit, but designing that will take some knowledge...
    – Solar Mike
    Jul 9, 2020 at 17:03
  • While I agree with most of what you say, I don’t see what you are saying about the polarity sensitive issue with an LED. If the anode is connected to the battery positive, when would the cathode go more positive than the battery positive. If this is possible, then a series diode could be connected to block the reverse current.
    – HandyHowie
    Jul 10, 2020 at 13:10

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